1. Field of the Invention
This invention relates generally to the field of seismic data processing. Specifically, the invention is a method for transferring seismic horizon interpretations between three-dimensional volumes.
2. Description of the Related Art
An important trend in the petroleum exploration and production industry is the increased desire to rely on seismic data to guide appraisal and development following discovery. This, in turn, results in the generation of a greater number of versions of the seismic data volumes that result from the acquisition and processing of seismic data. For example, at least one, and often several, seismic surveys are required for exploration and prospect delineation. The data from each of these surveys may be interpreted in several different offset stack seismic volumes, and possibly reprocessed for specification of impedance and phase volumes. For the development and production phases, further seismic data acquisition is often needed, for example to acquire higher frequency data or for seismic time-lapse reservoir monitoring.
A common, virtually unavoidable, consequence of the seismic interpretation process is the shifting of the location of horizons between a reference survey and the different vintages of seismic data volumes that result from reprocessing later seismic surveys, or between 3D and 2D seismic surveys within the same area. The process of horizon transfer and alignment is often tedious, and the various techniques that are presently used, such as applying seismic mis-ties to horizons and horizon shifting and snapping, often result in an unsatisfactory correlation between the shifted horizons and the reference surveys. More specifically, a problem that must be addressed is the vertical time variance of the shift needed to align horizons along each seismic line. That problem can make single shifts inaccurate for multiple horizons or even for single horizons covering large areas.
One approach that has been employed in industry is simply to output the original interpretation and import it into the new volume data set. The original interpretation is then used as a guide and the horizon is re-interpreted in the new data set. This approach is computationally inefficient, and the re-interpretation requirement limits its usefulness.
Another approach that has been employed is to interpret a reconnaissance horizon grid and interpolate the grid to create a surface to use as a reference surface for the original horizon requiring a shift. The mis-ties are then computed, usually using a commercial product, and a static shift mis-tie is determined to apply to the original horizon. Limitations of static shift mis-ties constrains this approach. For example, one is that different seismic volumes have dynamic time mis-ties so that the correction of mis-ties for multiple horizons requires replication of the approach, whereas the horizon alignment approach compensates for dynamic time mis-ties for multiple surfaces between different volumes of seismic data applied from a single determination. Gridding of surfaces also introduces errors inherent to the gridding process, while the horizon alignment approach completely avoids gridding of surfaces.
A third common method to align a previously interpreted horizon to a new version of the original seismic data is to copy the horizon to the new seismic data, estimate the shift for the horizon (typically determined through inspection by the interpreter), estimate a snap window for the shifted horizon, and then snap (a procedure that assigns a value at each trace of the surface to a user-specified seismic property or attribute of the trace, such as maximum and minimum values within a user specified time window) the horizon. The accuracy of the results of this procedure is in part dependent upon the snapping parameter choices that are made by the interpreter. The procedure can be accurate when the new seismic data does not significantly vary from the old version. More commonly, however, newly acquired data, and the processing and reprocessing of the original data, results in non-systematic misalignments between the new and reference data, thereby limiting the usefulness of this procedure.
Frequency (bandwidth) variations between the reference volume and the new volume(s) are also a common cause of horizon misalignment. Frequency differences, and differences in the migration velocities used in generating the volumes, often result in non-systematic misalignment. The complexity of this misalignment increases when other factors are added, such as seismic artifacts, different offset angle stacks, and when AVO analysis is performed, such as for Class 1, Class 2, and Class 3 amplitude anomalies. As will be understood to those skilled in the art, AVO amplitude anomalies are classified in terms of the local increase or decrease in reflection amplitude with varying offset angles, such as is caused by the varying impedances of adjacent geologic layers.
Other techniques have been used in the industry to tackle the problem. For example, the commercial seismic interpretation system Geoframe (IESX), of the GeoQuest division of Schlumberger Corporation, includes a MisTie analysis option. In this option mis-ties are calculated using either of two methods. The first method uses a statistical correlation approach between the seismic data at each intersection. The second method measures the mis-tie between interpreted horizons at each intersection. With either method, only a static shift is applied, and only one value per line intersection is permitted, whether it is from a single correlation or from an average of mis-ties from numerous horizons at that intersection. The user can specify whether a static shift is applied for all intersections, applied for some intersections, or a different shift value is applied to different intersections, and the corrections are applied to user-selected horizons.
There are a number of limitations of this approach. First, it is constrained to computation of constant (single static shift for a line) and/or variable (spatially varying) corrections, but cannot apply a dynamic shift to a seismic trace. The approach is therefore best suited for correcting static mis-ties. Second, the alignment for horizon transfer requires two intersecting surfaces to calculate mis-ties. Third, there is no ability to calculate a dynamic alignment along each seismic trace. Fourth, the alignment cannot be calculated between 3D volumes or between 2D seismic lines. Finally, the alignment technique does not utilize a time-shift (or lag) volume or associated correlation (confidence) volume output, which would contain dynamic shifts along each trace and allow, in effect, a volume of alignment corrections to be applied to all horizons and faults. The abstract of D. L. Brumbaugh, xe2x80x9cSMAP (Seismic Mistie Adjustment Procedure) Revisited and Revisedxe2x80x9d, 61st Annual SEG Int. Mtg., Houston, Nov. 10-14, 1991, Expanded Tech Program Abstr. Biogr. V1, pp. 332-334, applies a static shift to seismic lines to match other interpretations and well data. The SMAP revision allows for any orientation of seismic line to be optimally corrected, but does not accommodate dynamic shifts along a trace.
W. L. Walters"" U.S. Pat. No. 5,132,938, titled xe2x80x9cAdjusting Seismic Data to Tie Other Dataxe2x80x9d, issued Jul. 21, 1992, generates sets of data that are arranged according to the x-y-z coordinates of seismic lines or as isolated x-y-z points, such as well data. Different sets of data can be compared for the time gate about a common subsurface feature. Then a time delay is determined for each of the trace pairs from the different data sets. As will be understood to those skilled in the art, the terms time gate and time delay refer generally to interpretational differences which are more commonly referred to as mis-ties. These time delays are corrected to the reference surface using a least squares planar fit. The essential aspect of this method is that it requires a previously defined interpretation, whether seismic horizons or well data, to determine mis-ties. Thus, it requires a comparison of two sets of data to determine mis-ties. However, Walters"" patent does not show how to correlate volumes, rather than horizons, for misalignment before the horizons are transferred. Volume correlation would only require a single horizon interpretation in a reference volume. Volume correlation used for the horizon alignment actually does not require any horizons, but in comparison to the Walters"" patent, only one horizon, the original, is needed in order to transfer to another volume whereas Walters"" method requires comparisons between two sets of data. Further, Walters"" technique is a mis-tie technique within a single volume and not between different volumes.
The abstract of M. A. Herkomer and P. D. Whitney, xe2x80x9cMinimizing Misties in Seismic Dataxe2x80x9d, Battelle Memorial Inst. Computers Geosci., V 20, No. 5, pp. 767-795, 1995, describes a procedure that computes the mis-tied Z-value at each intersection to create a correction vector. The values of the correction vectors are smoothed and used to correct the mis-ties through a bulk shift (in other words a Z-axis correction). This technique, however, requires a comparison between two sets of interpreted surfaces and relies on the bulk shift for correction.
The abstract of F. Y. Michael, xe2x80x9cVintage Matching and Misties in Multisurvey Projectsxe2x80x9d, 65th Annual SEG Int. Mtg, Houston, Oct. 8-13, 1995, Expanded Tech Program Abstr. Biogr., pp. 499-500, makes several references to advantages in vintage matching using advanced workstation technology. The abstract references mis-ties related to bulk shifts, but not to volume shifting, but does specifically, outline the benefits involved in vintage matching.
Industry trends in seismic processing are driven toward increased use and manipulation of three-dimensional seismic data volumes. Improved and more efficient processing algorithms have enabled the generation of copious derivative volumes from what had been considered the basic set of seismic volume data. A growing concern in industry is the management of interpretation data between varying seismic volume types, and none of the above-described techniques involve a true 3D volume-based tool. In addition, none involve volume-based analyses, and none allow the straightforward transfer of existing interpretations to different volumes. It is evident that there will be an increasing need within the seismic interpretation community for such a tool. The present invention is directed to this need.
The present invention is a method for transferring seismic horizons between three-dimensional seismic data volumes. First, a first seismic data volume is selected, preferably as a reference data volume. Then a second seismic data volume is selected, preferably covering the same area as the first seismic data volume and containing the seismic horizons to be adjusted. Then the first and second seismic data volumes are time-aligned. This generates a time-shift volume. A seismic horizon is selected, preferably from the second seismic data volume. Finally, the time-shift volume is applied to the seismic horizon. This generates a time-shifted seismic horizon, which can be output as an adjusted interpretation of the originally selected seismic horizon.